small and cheap OTL schematic needed

[kenpeter]

Is abuse of grid as plate really any better than 4th circuit?
Plate in, grid grounded, cathode out... 1/(Mu-1) vs 1/(Mu)?
Either way, cathode emission puts upper limit on current.
"Impedance" isn't significantly different.

Dissipation in the grid is less than dissipation in the plate
for same flow of current. Due the shorter distance and
lower voltage, OTOH the plate is actually built to take it...

If you HAVE no choice but A2 to get max cathode emission,
then maybe grid as plate makes sense? Else I'm not seeing
why this cheat is better than increasing plate voltage to
obtain the same emission?

 

[desperateaudio]

An example of a more conventional simple OTL is the design by Tim Mellow, which appeared in the February issue of Audio Express. It's 25W per channel. Admittedly the output tubes (a pair of 6C33C's per channel) could hardly be called "small" (unless you compare them to transmitting tubes or something like that!), but the design is about as simple as they get. I have built one, and it sounds great. Another nice design, which I've also built, is the one by Hans Beijner, again using 6C33C's. Both of these are essentially modified Futterman designs. I've built a circlotron too, using a design by Alan Kimmel, using EL509S tubes in the output. This one is a bit more complicated, because of the need for two separate floating output power supplies per channel.

Interestingly enough, it is really rather easy to get a low output impedance with an OTL, particularly with the use of negative feedback. A fraction of an ohm output impedance is quite routine for such amps.

Chris

Thx Chris. I am thinking about building Tim's amp and considering using 4 6C33C's as output for some more power. It seems that at the 200ma boundry the front end should be able to push 4 tubes. AGREED? Would love to hear opinions. I am planning mono blocks